This is a major change, where Zsh modules/plugins are not git submodules
in the Zim repo anymore, but customized and installed separately as
individual repositories. The discussion about this started more than 2
years ago in #88. Closes#299.
This will allow contributors' modules to live in their own repositories.
See #33, #138, #262, #281, #324.
The current code has what, up to this point, I considered to be the best
balance between simplicity, execution speed and number of files.
One measured decision was to make the initialization of modules depend
only on the `':zim' modules` style, keeping it as fast as possible.
The `':zim:module' module` style is used to install, update and clean
the modules, all operations that happen after the user got his
as-blazing-fast-possible shell prompt.
Even though I didn't care much about making install or update fast,
`xargs` has a nice feature of allowing commands to be executed in
parallel with `-P`. I took advantage of that.
I've also worked on making the `zimfw` utility give the user some nice
(while still minimalistic) output. Also I'm suggesting this as the new
name for the `zmanage` tool, since `zimfw` does not shadow the `zim`
wiki tool.
